Aqueous coating compositions comprise a principal resin, and may contain a crosslinker, pigments dispersed in a grind resin and other additives such as solvents, control agents, fillers and the like.
Pigments are typically dispersed in a coating composition by means of a pigment paste. Pigment pastes are usually prepared by dispersing a pigment in a grinding resin in the presence of plasticizers, wetting agents, surfactants or other ingredients in a ball mill, sand mill or continuous mill, until the pigment has been reduced to the desired particle size and is wetted by the resin or dispersed in it. Such methods require lengthy processing times and yield dispersions having relatively poor color development and stability, compared to solvent-based paints.
Dispersing agents for dispersing pigments and dyes in organic liquids are disclosed in U.S. Pat. No. 4,937,014, to Canestri, issued Jun. 26, 1990. This composition contains an amino acid that has first been reacted with a metal salt and then is reacted with a carboxylic acid terminated polyester.
The present invention is directed to a pigment dispersant composition for inorganic pigment usable in an aqueous coating composition. The invention provides dispersions containing a salt of a transition metal or rare earth metal. The dispersant compositions are highly effective for dispersing inorganic pigment and demonstrate unexpected results by decreasing the viscosity of the dispersant composition containing the inorganic pigment. As a result of the decreased viscosity a higher concentration of pigments can be used in the dispersion, resulting in a higer pigment to binder ratio. Due to increased solids content, the volatile content of the pigment dispersion is decreased.
When the dispersant composition of the present invention is used with surface coated metal flake pigments, particularly with aluminum flake pigments, exceptional results are obtained for decreased corrosion of the pigment in an aqueous coating composition. This is significant because the water-borne coating compositions in use today have a basic pH. The pH of acrylic coating compositions typically ranges from 8.0-9.0, and the polyurethane coating compositions typically have a pH ranging from 7.5 to 8.0.
Exposure of metal flake pigments to a basic pH aqueous environment, results in the formation of metal hydroxide ions, which are soluble in water. The production of metal hydroxide ions results in the solubilization of the metal pigment and the exposure of more pigment surface area to the corrosive environment. The formation of metal hydroxide occurs relatively rapidly with metal flake pigments due to the high surface to mass ratio of the small particles. The reaction is a form of corrosion and converts the pigment to a hydrated oxide form unsuitable for pigment use, as it destroys the metallic pigmentation properties of the mirror-like particles. Corrosion is exacerbated since the reaction of the metal in water results in the continuous formation of H.sup.+ ions, evidenced by the production of hydrogen gas, and OH.sup.- ions. The H.sup.+ ions attack and corrode the metal pigment particles and the OH.sup.- ions cause the pH of the environment to further increase. The high pH of water-borne coating systems speeds up the reaction that produces the hydrated metal oxide and results in serious degradation or corrosion of metallic flake pigment used in metallic coatings. Contact with the environment is continuous over extended periods of time, since coatings containing the pigment are often stored for 6 months or more before application.
It has been found that the transition metal and rare earth metal salts inhibit corrosion of the metal flake pigment particles, particularly aluminum flake pigment particles, in the basic pH aqueous coating environment. This is due to a compact film of transition metal or rare earth metal oxides and hydroxides that replace the natural oxide film on the surface of the metal flake pigment. It is hypothesized that the transition metal or rare earth metal oxide/hydroxide film forms at local cathodic sites on the surface, where the alkaline conditions generated by oxygen reduction reactions cause the metal oxide to dissolve and the transition metal and/or rare earth metal oxide to precipitate.
The present invention is also directed to an aqueous coating composition containing the pigment dispersant compositions and an article coated with the coating composition.